1. Field of the Invention
The invention relates generally to borehole logging utilizing shear and/or torsional wave propagation to derive data relative to acoustic wave transit times in formations adjacent the borehole.
2. Description of the Prior Art
The prior art includes numerous types of acoustic energy logging tools that may be used for data collection along a borehole. The previous shear wave logging tools have not been designed for the sole detection of shear wave travel times along the vertical dimension, and most of the acoustic logging tools used today need mud or water-filled holes in order to function. Presently used logging tools have two opposed transmitters and two opposed receivers thereby to eliminate hole caving effects and miscentering of the tool. The magnetostriction-type transmitting transducers generate pulses on the order of 17 to 25 microseconds in width at a rate of around 20 times per second. Hence, compressional waves are created in the borehole fluid which in turn generate formation compressional as well as shear, fluid, and boundary waves. The detected signals then reflect this character as the returned signal indications show compressional and shear waves from the formation, and boundary and fluid waves of greater or lesser travel times. Thus, shear wave velocities in the formations surrounding boreholes can only be determined by either electronically or visually sorting the shear wave arrival time at the detectors from the multitude of other waves that are also received at the detectors.
U.S. Pat. No. 3,794,976 in the name of Mickler teaches the generation of high frequency shear waves for transmission through adjacent formations with subsequent detection to derive information about the borehole wall and the immediate formations therearound. This teaching utilizes a short burst of relatively high frequency acoustic energy as input to the formation with detection in the same horizontal plane through the borehole. The return signals derived then provide measurements which may be likened to characteristic anomalies such as fractures in the earth formation immediately adjacent the borehole. Another U.S. patent in the name of White, U.S. Pat. No. 3,683,326, teaches an echo ranging device utilizing vertically displaced detectors and transducers, the data from which may be further processed to detect certain inhomogeneities located near the borehole. The method and apparatus of this invention are directed toward cancellation of interfering fluid-borne signals and other direct transmission signals through detector polarity discrimination by signal combination.